my 6.4 has 3.73 ratio and I've been thinking about switching them out. the first three gears run out quick when i get on it and also its sitting at about 2000 rpm at 70-75, the gears are great for towing but it would be nice to have a little lower rpm on the highway. Does anyone have the zf6 with 3.55 or 3.31, how do you like it, how does it tow, how is the daily driving around town, on the track? Thank you for any input guys it is appreciated

I THINK, not 10% sure, that the ZF6 trucks only came with 3.73 or higher gears.
This site: Ford Vehicle PDF Brochures has a PDF of the sales brochures back to 08 available and I looked in there and I did not see anything mentioned as to rear axle ratio other than the Tow Boss package having 4.30s. I will search around and see what I can find...
Also, my auto has 3.73s

EDIT: Found a listing of what appears to be all optional rear ratios. The 3.73 open diff was the standard equipment with limited slip available as an option in 3.55, 3.73, 4.10, and 4.30 I do not know the accuracy of this information though as I cannot remember hearing of anyone with 3.55s in anything stock before 2011

Thank you for lookin man i guess i'll just have to try and see how it works, and btw the stock 20"s painted black look awesome on your truck i think i might have to do that when i get back! and about the window sticker thing have you tried looking up your vin online i did that with an old truck and got a new window sticker i'll see if i can find the site

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